1. Field of the Invention
The present invention relates to an arrangement for supporting a rocker shaft in an engine.
2. Description of the Related Art
Four-stroke engines are provided with a valve-moving device within the cylinder head. The main component members in such valve-moving devices are a cam shaft, which rotates in motion with a crank shaft, an intake valve and an exhaust valve, valve-moving intake and exhaust cams provided on the cam shaft, intake and exhaust rocker arms, which transmit the rotational motion of the intake and exhaust cams to the two valves, a rocker shaft supporting the rocker arms with freedom of rotation, and the like.
A known method of supporting the rocker shaft within the cylinder head is, by way of example, to provide bearing bosses, which support the cam shaft with adequate excess materials, to run to an insertion hole, which is parallel with the cam shaft through the excess material, and to pass through a rocker shaft through this run-through hole.
In addition, normally a single rocker shaft is used for both the intake and the exhaust rocker arms.
Rocker shafts require an extremely high degree of precision in cases where a run-through hole for the rocker shaft is machined into the excess material of a bearing boss since any discrepancy in positioning has a major effect on the performance of the valve-moving device, not least in that it produces variations in the valve timing.
However, high-precision machining is undesirable since it requires special equipment and the machining itself entails costs.
Further, providing a bearing boss with excess material for supporting a rocker shaft also produces problems in that it causes increased weight, and pores are liable to occur during casting if a large amount of excess material is formed.
Meanwhile, if a single rocker shaft is used for both the intake and the exhaust rocker arms, there are limits on factors such as the position in which the valves are installed, and the degree of freedom in the layout of the valve-moving device is reduced.